1. Field of the Invention
The present invention relates to an image forming apparatus which includes a waste-liquid discharging mechanism which facilitates functional recovery by discharging an ink from nozzles formed in a recording head, and a method of discharging ink from the recording head.
2. Description of the Related Art
In an image forming apparatus of an ink-jet recording type, a recording of characters and/or images is performed by making liquid droplets of ink fly toward a recording medium such as a plain paper, thereby making the ink adhere to a surface of the recording medium. After the ink is supplied from an ink cartridge in which the ink is stored, to a recording head, the ink is jetted as fine droplets from a plurality of nozzle groups which are formed in a nozzle surface of the recording head. Accordingly, when there is a defect in ink jetting due to drying of the ink at an opening of each nozzle, blocking due to adhering of dust, and/or an air bubble which has entered the nozzle, it leads to a decline of an image quality.
Therefore, a recovery operation for a function of the recording head is carried out by discharging the ink periodically from the nozzle by a waste-liquid discharging mechanism which is provided on one portion inside a main-body case for eliminating such factors causing the defect in ink jetting.
This waste-liquid discharging mechanism includes a plurality of suction caps which are provided for each of the nozzle groups, and which cover nozzles in each of the nozzle groups by making a contact with a nozzle surface of the recording head, a common suction-force generating mechanism which generates a suction force in these suction caps, and an ink discharge channel which connects the suction caps and the suction-force generating mechanism. The ink discharge passage includes a plurality of first channels, one end of which communicates with each of the suction caps, and a second channel which joins the other ends of the first channels and communicates with the suction-force generating mechanism.
The suction caps are brought into contact with the nozzle surface of the recording head, and the each nozzle group is covered by respective suction cap, and the suction force is generated in the suction cap. By generating the suction force in the suction cap, the ink is sucked from each of the nozzle groups. The ink sucked in the suction cap by the suction-force generating mechanism is transported to a waste-liquid discharge tank after being gathered in the second channel upon passing through the first channel communicating with each suction cap.
In an image forming apparatus which includes such waste-liquid discharging mechanism, there occurs to be a difference in an amount of ink flowing through the first channels according to a difference in a nozzle diameter, the number of nozzles, and/or a channel resistance of the nozzles. For example, it may occur such phenomena that only ink in a first channel having a low channel resistance and a high ink flow rate flows into the second channel, and ink in a first channel having a high channel resistance and a low ink flow rate is hardly discharged. Therefore, some of the nozzle groups may not be recovered sufficiently and moreover, some defects such as discharging unnecessarily ink may occur.
Therefore, to eliminate such defects, the following image forming apparatus has been proposed. In this image forming apparatus, a substantial suction force is generated in nozzle caps covering a nozzle group in which a resistance acting on the ink flowing through the nozzle (channel resistance) is high due to a small nozzle diameter or a large number of the nozzles, whereas a low suction force is generated in nozzle caps covering a nozzle group in which the channel resistance is low due to a large nozzle diameter etc.
Concretely, a discharge hole of the suction caps covering the nozzle group having a low resistance for the ink flow inside the nozzle of which diameter is large is made small, and a diameter of a tube which forms the first channels is made small. Moreover, a length of a discharge tube of the suction caps covering the nozzle group having a large nozzle diameter are made to be longer than a length of a discharge tube of the suction caps covering the nozzle group having small nozzle diameter (For example, refer to pages 2 and pages 4 to 6, and FIG. 4 of Japanese Patent Application Laid-open No. 2004-249631). In Japanese Patent Application Laid-open No. 2004-249631, two suction caps are considered to be formed integrally.
By adjusting an amount of ink sucked of each of the nozzle groups by changing the suction force in such manner, a difference in time required for the functional recovery of each group is reduced, and the functional recovery of the recording head is facilitated without wasting the ink.